[time-nuts] Question about UT1 and the IERS Reference Meridian

mflawson2 at cox.net mflawson2 at cox.net
Tue Apr 28 23:06:02 EDT 2015


Okay, I've tried to research this for a few days, and seem to be running 
into conflicting data.

Some articles say that UT1 is based on the IERS Reference Meridian (IRM). 
Other articles say that UT1 is based on mean solar time at the Prime 
Meridian (Greenwich).  It can't be both!  Which one is it?   In other words, 
which meridian would I need to stand on to indicate Solar Noon as 
12:00:00.000 (UT1) on a day of the year where the equation of time is 0 
seconds offset?

>From what I can reckon, a 200-400 millisecond difference exists between the 
two longitudes, which are separated by about ~102 meters at Greenwich.  So, 
if UT1 (and hence UTC) is based on the IRM and not the Prime Meridian, then 
at some point did clocks have to be adjusted ~200 milliseconds "away from" 
the Prime Meridian when the IRM was defined?  Or was it sloppy and they 
treated the two as one and the same?

Anyone know? 




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